Catalytic domain of the Protein Serine/Threonine Kinase, MAP/ERK kinase kinase 4. Serine/threonine kinases (STKs), MAP/ERK kinase kinase 4 (MEKK4) subfamily, catalytic (c) domain. STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. The MEKK4 subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. MEKK4 is a mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK or MKKK or MAP3K), that phosphorylates and activates MAPK kinases (MAPKKs or MKKs or MAP2Ks), which in turn phosphorylate and activate MAPKs during signaling cascades that are important in mediating cellular responses to extracellular signals. MEKK4 activates the c-Jun N-terminal kinase (JNK) and p38 MAPK signaling pathways by directly activating their respective MAPKKs, MKK4/MKK7 and MKK3/MKK6. JNK and p38 are collectively known as stress-activated MAPKs, as they are activated in response to a variety of environmental stresses and pro-inflammatory cytokines. MEKK4 also plays roles in the re-polarization of the actin cytoskeleton in response to osmotic stress, in the proper closure of the neural tube, in cardiovascular development, and in immune responses.